The present invention relates to electrical protection apparatus and, particularly, to resistors for protecting high voltage electronic equipment from the harmful effects of electrical discharge within such equipment.
Modern x-ray equipment includes an x-ray tube which is energized by voltage of 50-100 thousand volts or more to create desired x-rays. The x-ray tube normally presents a high impedance of several hundred thousand ohms to the applied voltage, resulting in a relatively small current flow. Occasionally an electrical discharge occurs within the tube, drastically reducing its impedance, thereby increasing the current which flows through the tube. The large current occasioned by such a discharge can damage or shorten the life of the x-ray tube. This is a particularly serious problem since x-ray tubes may cost in excess of $30,000, and replacing them can result in large repair charges. Given the expenses involved, it is desirable to protect high voltage tubes such as x-ray tubes from damage caused by internal discharges.
It has been found that a low inductance resistor can be placed in series with the high voltage input to protect a high voltage tube from discharges. The resistor value is chosen to be relatively low with respect to the normal tube impedance, and to be substantially equal to the resistance of the high voltage source and its attaching conductors. When a tube including the protection resistor discharges, a substantial part of the applied voltage is dropped across the protection resistor, quenching the discharge and protecting the x-ray tube from damage.
The protection resistor should exhibit low inductance, have sufficient power handling capability to quench the discharge without damage to itself, and be of a relatively small size to permit connection of the resistor at a point as near to the high voltage tube input as possible.
Low inductance high power resistances are known in the art and as described, for example, in U.S. Pat. No. 3,360,905 to Rietz, et al. The Rietz, et al. patent discloses an arrangement in which a plurality of individual resistors are spirally wound, physically placed on top of one another, and serially connected to provide the needed resistance. The resistances are wound so that each layer spirals in a different direction than the layer or layers it is next to. Such winding and physical placement substantially reduces the inductance of the overall assembly. The resulting resistor is, however, large and not suitable for connection in existing x-ray equipment.
A need exists for a low inductance protection resistor which is of small size and capable of direct connection to existing high voltage tubes.